Commercial inverters, and specifically pulse width modulation (PWM) inverters, are often utilized for driving induction motors, and other industrial and domestic appliances. Referring to FIGS. 1A and 1B, a commercial inverter 10 and an induction motor 11 form a basic application configuration in industry. The inherent switching mode operation of these inverters leads to a plurality of problems caused by unwanted harmonics, which are the subject for minimization.
In this inverter-motor system, PWM operations of the inverter 10 generate large conducted emissions through the power leads which exceeds the Federal Communications Commission's (FCC) and International Special Committee on Radio Interference (CISPR) of International Electrotechnical Commission (IEC) limits on conducted emissions for digital devices by approximately 25 to 30 DB.mu.V in the frequency range from hundreds of KHz to tens of MHz. The large impulse emission currents also cause waveform distortion of the line voltages, especially on the line to ground voltages and appear also in the line currents. The depth of the impulse notches could exceed 20% of the sine wave voltage. R.F. leakage currents that flow to the ground from the motor's frame or the inverter's enclosure produce a zero sequence current both at the input power leads 12 of the inverter 10 and at the output power leads that also generate harmful radiated emission.
The problems involved in harmonic control over the power frequency ranging up to the 35th order are already well addressed and an application guide has been provided in IEEE Std. 519-1992. However, in the case of Radio Frequency Interference (RFI) associated with the use of static converters, especially for the case of power drive systems, there has been no mandatory test requirement governing the unwanted noise produced nor has the problems concerned been well documented. On the other hand, the subject of Electromagnetic Interference (EMI) or RFI for information technology equipment have been an important aspect of those product's performance because Electromagnetic Compatibility (EMC) regulations for these devices have existed in most major countries. Digital devices having clock frequencies that exceed 9 kHz cannot be legally sold in the U.S. unless they have been tested and found not to exceed limits on radiated and conducted emissions set by the Federal Communications Commission (FCC). Also, countries in Europe have imposed similar requirements on digital devices. One of the most important standards setting organization for commercial EMC is CISPR, a committee well known as the International Special Committee on Radio Interference, which develops recommended EMC test limits and test procedures. Most countries use CISPR standards for their own regulations. The European Community (EC) approach has been to develop a common set of EMC requirements, which are collectively known as the European Norms (EN's) and are largely on CISPR and IEC standards.
It can now be said that the subject of EMC is of worldwide concern to manufacturers of digital products as it is a critical aspect in the marketability of their products. Thus far, concerns have focused mostly in low power level applications of digital devices as the FCC has temporarily exempted five subclasses of digital devices from meeting the technical standards of the requirements in 1987. One of these classes concerns industrial control systems used in an industrial plant, factory, or public utility. However, this exemption does not imply that EMC problems can be neglected in motor drive systems. It is generally recognized that the exempted devices are still subject to the general conditions of operation such that no harmful interference should be generated or that interference must be accepted that may be caused by the operation of an authorized radio frequency device. Also since the exemptions mentioned are temporary, the rules can be revised forcing major design problems for equipment manufacturers not designing for low EMI. In addition, with the new standards (European Norm) for inverter drive systems being proposed by CISPR, the subject of EMC has become a critical issue for inverter manufacturers to insure their products have good EMC performance and operate properly in their intended environment.
Although the subject of EMC is well documented on low power digital devices, applications of digital techniques in the higher power ranges concern situations different than with a signal processing system. Some issues particular to high power switching are important in establishing to what degree the power electronics system would produce and interfere with nearby electrical or electronic devices or systems.
Therefore, different measures have been undertaken in order to minimize conducted emissions. For example, U.S. Pat. No. 4,673,851 discloses a PWM DC motor operating system with RFI suppression in which a freewheeling diode is connected across the motor armature terminals and is switched by a FET. The switched current path comprises, in series, a DC power supply, a low pass filter including charge storage means, a DC motor armature, a field-effect transistor (FET) and a resistor. A capacitor is connected across the FET as well as a diode with minimal lead length to reduce stray inductance. The capacitor is effective to communicate voltage shifts across the resistor with FET switching to the cathode of the diode and thus serve as a source of supplemental current flowing through the diode during diode switching partially in preference to the charge storage means of the low pass filter, whereby the diode switching voltage spikes communicated through the low pass filter to the DC power supply are reduced in amplitude and frequency.
U.S. Pat. No. 5,103,147 proposed an operating device for consumers connected to the electric system of mobile units in which an interference eliminate circuit including a series coil and at least one capacitor for high capacitance is arranged between the electronic control system and the electric system of the mobile unit. The series coil is equipped with an iron-power core and designed in such a manner that no saturation is obtained even under maximum current condition so that a sufficiently high inductivity reserve remains available for pulse damping.
U.S. Pat. Nos. 4,800,478, 4,977,492, 4,994,951, 5,001,619, 5,053,939, 5,053,939, 5,065,304, 5,224,028 and 5,235,503 concern means for controlling PWM inverters with purpose of suppressing switching harmonics. For example, in U.S. Pat. No. 04,977,492, an inverter controller adjusts a selected switching pattern to accommodate minor variations and loads in the neighborhood of that pattern. The controller realizes in real-time the equivalent of an iterative solution of predetermined patterns. A number of representative pulse wide modulated (PWM) patterns are selected for a range of loads and link ripples. A portion of fundamental output is inserted onto the DC link to control one or more higher harmonics.
All these efforts will not be effective to suppress the RFI emissions caused by switching transient, voltage or current change rate, e.g. dv/dt or di/dt.
Traditionally, AC power line filters and isolated transformers are the normal means for the suppression of conducted emissions from digital devices in low power applications. However, these filters have restrict applications in higher power range due to their inherent disadvantages, such as extra volume and cost for installation; a strict grounding condition need for proper choice of insertion loss for common mode and differential mode with a specific application.
Also, since efficient suppression of EMI should be carried on the basis of understanding the sources causing EMI and suppressing it where it occurs, AC power line filters, being constructed independently, are not always competent to realize this criterion.
Therefore, it would be highly desirable to provide an industrial inverter with cost-effective and low-weighing means for EMI suppression.